ADSL communications rates are normally established between a customer's modem and the central office equipment based on initial line conditions and a subscribed to service rate. This initial line rate normally remains fixed unless poor line conditions interrupt service or communication with the central office is interrupted for some other reason, e.g., as part of provisioning a new service rate. Any re-synchronization results in a disruption of service. Re-synchronization due to poor line conditions can result in a ratcheting down in the provided line rate over time. To provision a higher service rate, e.g., in response to a subscriber request for a change in service, a full initialization operation is normally performed which disrupts any ongoing communions sessions being conducted over the line. Thus, in current systems, line rates are not changed on-the-fly and a service subscriber must subscribe to a rate which supports the most demanding application used by the subscriber even if the high bandwidth application is used only on a sporadic basis or face interruptions in service each time a rate change is made.
Problems with existing DSL will become clear if one appreciates the existing setup and resynchronization process. In Asymmetric Digital Subscriber Line (ADSL) systems, a user's modem at a customer site, e.g. an ADSL Termination Unit-Remote (ATU-R), when powered on, attempts to establish communications with a modem at a central office, e.g. a Digital Subscriber Line Access Multiplexer (DSLAM) Line Port, ADSL Termination Unit-Central office (ATU-C).
Known initialization sequences, used to establish communications between a central office's DSLAM modem, an ATU-C, and a Customer Premises Equipment (CPE) modem, an ATU-R, in the ADSL environment are described below. These initialization sequences are sometimes referred to as the training-up of the modem or ‘showtime’. The series of events comprising the initialization sequence may be initiated due to any of the following conditions.                1. The initial power-up of the ATU-R.        2. Subsequent power-up or reboot of the ATU-R.        3. Loss of power to the ATU-R and subsequent restoration of power to the ATU-R.        4. The initial power-up; reset or re-provision of the ATU-C.        5. Loss of sync between the ATU-C and the ATU-R.        6. Loss of power to the ATU-C and subsequent restoration of power to the ATU-C.        7. Replacement of the DSLAM Line Card with ATU-C ports and initialization of the Line Card.        
The drawing 100 of FIG. 1 describes a known initialization sequence used to establish communications, in an ADSL environment, between a central office DSLAM modem ATU-C 102 and a CPE modem ATU-R 104 for the ANSI T1.413 mode of operation. The ADSL initialization signals can be described in terms of frequencies or, equivalently, in terms of the tone indices representing those frequencies, where the frequency of tone k is k*4.3125 kHz. ATU-C 102 is in a quiescent state, C-QUIET 106, monitoring for an activation signal from ATU-R 104. ATU-R 104 generates an initial activation request signal, R-ACT-REQ 108, defined as the k=8 tone (34.5 kHz), sends signal R-ACT-REQ 108 to ATU-C 102, and then enters a quiescent monitoring state, R-QUIET 110. ATU-C 102 receives the R-ACT-REQ 108 signal and generates a response signal, C-ACT 114. The C-ACT 114 signal is used to select timing modes, and may be one of the following signals: C-ACT 1 k=48 tone (207 kHz), C-ACT2 k=44 tone (189.75 kHz), C-ACT3 k=52 tone (224.25 kHz), or C-ACT4 k=60 tone (258.75 kHz). ATU-C 102 sends the selected C-ACT 114 signal to ATU-R 104, and enters a quiescent monitoring state, C-QUIET2 116. ATU-R 104 receives C-ACT signal 114 and generates an acknowledgment signal R-ACK 118. The R-ACK 118 signal acknowledges reception of the C-ACT signal 114, and selects some additional parameters, where R-ACK 118 is one of the following signals: R-ACT1 k=10 tone (43.125 kHz) or R-ACT2 k=12 tone (51.76 kHz). ATU-R 104 sends the selected R-ACK signal 118 to ATU-C 102, and then enters a quiescent monitoring state R-QUIET2 120. ATU-C 102 receives the R-ACK signal 118, generates an acknowledgement signal, C-REVEILLE 122, which is the k=56 tone (241.5 kHz) tone, and sends C-REVEILLE 122 to the ATU-R 104. ATU-R 104 detects the C-REVEILLE signal 122, waits until C-REVEILLE 122 has finished, and then sends R-REVERB1 signal 124. R-REVERB1 124 is the first signal in the initialization sequence that is not a single tone. The exact tones (subchannels) used to generate R-REVERB1 124 are vendor discretionary, and may be from 1 to 31. To ensure that the R-REVERB1 signal 124 has the same power spectral density (PSD) mask as the ADSL upstream data, the most common tones begin at tone 6 or 7 and end at tone 30 or 31.
A known initialization sequence is used to establish communications in an ADSL environment between a central office DSLAM modem, ATU-C 102, and a CPE modem, ATU-R 104. The G.dmt (G.hs) mode of operation shall now be described. G.hs uses 3-tone redundancy for robustness of ADSL modem training. The ATU-R 104 sends tones k=9, 17, 25 to indicate that ATU-R 104 can operate in ADSL over POTS (G.dmt Annex A). These three tones (k=9, 17, 25) are known as the A43 upstream carrier set. The ATU-R 104 may also send tones k=37, 45, and 53 to indicate that it can communicate G.hs information over these carriers as well. These tones (k=37, 45, 53) are the B43 upstream carrier set. ATU-C 102, which has been monitoring for a request from ATU-R 104, receives the upstream carrier set(s) and responds with a downstream carrier set(s) of tones. Tones k=40, 56, 64, which are the A43 downstream carrier set, indicate that ATU-C 102 can operate in ADSL over POTS. Also, the ATU-C 102 may send tones k=72, 88, 96, which are the B43 downstream carrier set, to indicate that it can communicate using this carrier set as well.
Modems 102, 104 then communicate, negotiate capabilities, and train the transceivers using the negotiated parameters. The negotiation of capabilities may involve: sending test signals on the DSL line, measuring of Signal-to-Noise-Ratio (SNR) and Attenuation (ATN) at the receiving end of the line, requesting test parameter updates, obtaining initial values for maximum possible achievable data rates on the DSL line between modems 102 and 104, transferring initial subscribed data rate information to ATU-C 102 from a provisioning system, calculating an initial assigned data rate value, and transferring of the assigned data rate value from ATU-C 102 to ATU-R 104.
The ITU G.992.1 G.DMT standard, which may be used by DSLAM modem ATU-C 102 and CPE modem ATU-R 104, divides frequencies into 255 sections, or bins. The separation between each bin is 4.3125 KHz. Each bin can carry 0 or 215 bits, with the resulting possible bit rates in increments of 32 Kbps. The downstream data traffic, from Central Office DSLAM Line port modem ATU-C 102 to the user's CPE modem ATU-R 104 uses bins 37 to 255, or 159.562525 to 1099.6875 KHz. The upstream data traffic, from the user's CPE modem ATU-R 104 to the Central Office DSLAM modem ATU-C 102, uses bins 6 to 29, or 25.875 to 125.0625 KHz. Noise and other interference on the line cause attenuation on the line, which in turn decreases the Signal-to-noise ratio, SNR. During initialization, based upon provisioning and determined maximum achievable data rate, the modem ATU-R is assigned data rates designating which bins may be used for the upstream and downstream directions.
In known systems, once a CPE modem ATU-R 104 is powered on, it periodically sends out a tone (R-ACT-REQ) 108 in T1.413 and/or multiple tones (upstream carrier set(s)) in G.hs, in an attempt to sync to the ATU-C 102, as previously described. Once sync has been established between modems ATU-R 104 and ATU-C 102 and the initial provisioning has completed, resulting in an assigned data rate value, the ATU-R 104 may communicate wit1 1 the ATU-C 102 at that initial assigned data rate value. Communications may be interrupted by the following fault or termination conditions: a power loss at either end, the threshold of the sync margin is negatively impacted, or the SNR falls below an acceptable level, e.g., due to changing line conditions. In any of these fault or termination scenarios, including the low SNR case, the modem, ATU-R 104 will attempt to re-sync with the ATU-C 102, e.g., at a new lower data rate.
Unfortunately, the re-synchronization process causes a disruption of service. If the re-sync was triggered by an unacceptable low SNR, typically the new initial assigned data rate value established during the re-initialization process will be lower than the previous data rate. Thus, over extended periods of time, the re-synchronization process often results in a ratcheting down in terms of the data rate since the re-sync process is triggered in response to decreasing line condition quality but not improvements in SNR. This is understandable since the current process involves disrupting any on-going communications session when resynchronization is preformed to implement a data rate change.
Thus, in known systems, there is no method today to gracefully decrease the assigned data rate to adapt to changing line conditions without a disruption in service, i.e., the resynchronization process used at start up is used each time SNR falls to an unacceptable level.
In existing systems, if the line conditions are insufficient during initialization, to set the assigned data rate at the subscribed to, e.g., provisioned, data rate, the assigned data rate is set at a lower supported data rate, and the user is limited to that lower assigned rate as a ceiling data rate for the communications session. With known systems, there is no attempt to increase the assigned data rate if the line conditions should improve during operation, and there is no method today to increase the assigned data rate without going through an initialization sequence which results in a disruption of service.
Users of DSL lines, at different times, require different levels of service, e.g., based on the application currently in use. For example a user may primarily use the DSL line for voice and require a relatively low data rate; however, occasionally, for short intervals, the DSL line may also be used for video conferencing requiring a relatively high data rate. In order for the user to satisfy his needs, he could continuously subscribe to a higher level of service than he generally needs; however, this approach is inefficient since during most of the time, the user would be wasting bandwidth, e.g., by paying for bandwidth that goes unused. It would be advantageous if service level changes could be requested and implemented on demand, to supply additional bandwidth when needed by a specific application and then remove the additional bandwidth when the application terminates without interfering with other on-going communications sessions which are terminated in existing systems.
Based upon the above discussion, it is clear that there is a need to have a dynamic and seamless adjustment capability for controlling DSL modem rates without the need to perform a complete initialization/resynchronization process which would interfere with ongoing communications sessions. In particular, there is a need for supporting downward rate adjustments in the case of a worsening of line conditions, e.g., before synchronization is lost. There is also a need for allowing a line rate to be increased in response to improved line conditions without requiring re-synchronization processes which would interfere with existing communications sessions. In addition, there is a need for a method which would allow a DSL rate to be changed, e.g., in response to changes in a subscriber's services needs, without interrupting existing communications sessions.